Knowing that the coordination number of both Fe2+ and Co2+ cations is 6, I am unsure which ionic radius I should use as one pertains to a low spin and the other is a high spin state. My guess is that I would use the high spin state ionic radius for both of these cations because it is octahedral in geometry in the crystal lattice, but I'm not too sure.

My guess is that I would use the high spin state ionic radius for both of these cations because it is octahedral in geometry in the crystal lattice, but I'm not too sure.

Octahedral geometry by itself doesn't tell you much. (Tetrahedral complexes, on the other hand, are almost always high-spin, because of the small field splitting). For octahedral species, you have to look at where the ligands fall in the spectrochemical series. In this case, chlorides are weak ligands, so the splitting will be fairly small, and you'd expect high-spin complexes.